i may have missed something, so i ask this in seriousness, not to argue........but just how do we know co2 has a warming effect?
Controlled experiment: isolate a gas, see what happens when you subject it to thermal infrared radiation.
This is actually a good question.
The "green house effect" is caused by the way radiation transfers through the atmosphere. Different kind of gases are opaque to different wavelengths of light in a very complex way. Tracking the energy flow through the atmosphere (influx from the sun heating and out flux from the earth cooling) is radiative transfer. The spectrum changes as it goes through the atmosphere and calculating this is a very difficult business both in terms of computing power and interms of the physics of matter-radiation interaction involved.
The layman argument is that CO2 is mostly transparent to the sunlight but absorbs and reflect IR, therefore it lets the sun heat the earth but does not let the earth cool by emitting IR. This is not oversimplification - it is just plane wrong. The net energy flux that goes in is exactly the same as goes out, otherwise the earth (or your green house) will heat forever. What it does is that it forces the spectrum to change in order to pass through. Yes, one way in which the spectrum changes is when the emitting body gets hotter. This shifts the spectrum into shorter wavelengths in which more radiation can pass. But there are many other effects that can suddenly make the atmosphere more transparent. For example: much of the absorption and re-emission of molecules happens in specific wavelengths (or "lines"). Once there is enough molecules to make the lines saturate (not letting any photon to pass), adding more gas has almost no additional effect. If the atmosphere itself gets warmer, the wavelengths at which the molecules absorb become "smeared" by the Doppler effect (the molecules are moving faster when hotter). They absorb in a wider range of wavelengths but less at each and suddenly spectral lines (emission at specific wavelength) that were completely absorbed can pass through, while re-emitted radiation by this gas has its spectrum blurred and shifted out of the absorption wavelengths of the next atmospheric layer. Now mix may of these gases together, in a varying vertical structure and try to propagate the spectrum through it.
A controlled experiment can give us the parameters we need for the quantum radiation-matter interactions, but will not help us solve the radiation transfer through the atmosphere, nor many many other secondary effects. Simplified computer models can give "good" approximations, but not good enough for the whole climate debate, where simplification induced errors are larger than the expected effect. Recent improved calculations with a MUCH better treatment of the radiation transfer show that most of the effect of increased CO2 is changing the vertical thermal structure of the atmosphere, but almost no effect at ground level even if the CO2 content is increased by factor 10. This however only refers to the specific problem of radiation transfer and disregards plethora of other issues.
